Considering vertical toolbars with buttons can be on the left or right, I'm not sure it's a focus or handedness issue. I would guess someone picked it and there

Considering vertical toolbars with buttons can be on the left or right, I'm not sure it's a focus or handedness issue. I would guess someone picked it and there was never a good enough reason to change it later. What is apparent though is that in the latest UI design trends people aren't even clicking on vertical scroll-bars enough to justify them being visible by default, and I presume they're mouse-scrolling instead, so perhaps left or right never really mattered much at all?